1. Technical Field
The present invention relates to an air/fuel ratio control device for an internal-combustion engine.
2. Background Art
Japanese Patent Application Laid-Open No. H09-126015 discloses an air/fuel ratio control device for an internal-combustion engine. In the air/fuel ratio control device, the amount of fuel which is injected from a fuel injection valve is increased when the state of the internal-combustion engine is in a high load region, which decreases the temperature of an exhaust gas discharged from a combustion chamber and suppresses thermal degradation in the catalyst disposed in an exhaust passage. In this way, in the internal-combustion engine in which the catalyst is disposed in the exhaust passage, there has been a demand for suppressing thermal degradation in the catalyst.
Incidentally, in the field of the internal-combustion engine, there is a known three-way catalyst capable of simultaneously purifying nitrogen oxide (hereinafter, this will be indicated by ‘NOx’), carbon monoxide (hereinafter, this will be indicated by ‘CO’), and hydrocarbon (hereinafter, this will be indicated by ‘HC’) which are contained in an exhaust gas at a high purification rate when the air/fuel ratio of the exhaust gas flowing into the catalyst is a theoretical air/fuel ratio. Then, as the three-way catalyst, there is also a known three-way catalyst which has an oxygen absorbing and discharging ability in which oxygen in the exhaust gas is absorbed to the catalyst when the air/fuel ratio of the exhaust gas flowing into the catalyst is leaner than the theoretical air/fuel ratio and oxygen absorbed to the catalyst is discharged therefrom when the air/fuel ratio of the exhaust gas flowing into the catalyst is richer than the theoretical air/fuel ratio.
Furthermore, as the internal-combustion engine in which the three-way catalyst is provided in the exhaust passage, there is also a known internal-combustion engine having a following configuration. When the air/fuel ratio of an air/fuel mixture formed in a combustion chamber (hereinafter, the air/fuel mixture formed in the combustion chamber will be simply referred to as an ‘air/fuel mixture’) is richer than the theoretical air/fuel ratio as the target air/fuel ratio, the air/fuel ratio of the air/fuel mixture is controlled so that the air/fuel ratio of the air/fuel mixture becomes leaner than the theoretical air/fuel ratio. On the other hand, when the air/fuel ratio of the air/fuel mixture is leaner than the theoretical air/fuel ratio, the air/fuel ratio of the air/fuel mixture is controlled so that the air/fuel ratio of the air/fuel mixture becomes richer than the theoretical air/fuel ratio. That is, in the internal-combustion engine, the air/fuel mixture with the air/fuel ratio leaner than the theoretical air/fuel ratio and the air/fuel mixture with the air/fuel ratio richer than the theoretical air/fuel ratio are alternately formed in the combustion chamber, whereby the entire air/fuel ratio of the air/fuel mixture is controlled to be the theoretical air/fuel ratio, and the exhaust gas with the air/fuel mixture leaner than the theoretical air/fuel ratio and the exhaust gas with the air/fuel ratio richer than the theoretical air/fuel ratio alternately flow into the three-way catalyst. In the internal-combustion engine, such air/fuel ratio control is executed so that the air/fuel ratio of the air/fuel mixture is controlled to be the theoretical air/fuel ratio as the target air/fuel ratio and the purification performance of the three-way catalyst is improved by exhibiting the oxygen absorbing and discharging ability of the three-way catalyst.
Further, there is also a known internal-combustion engine in which the three-way catalyst is provided in an exhaust passage and which has the following configuration. In an oxygen discharged state where the entire oxygen has been discharged from the three-way catalyst, the air/fuel ratio of the air/fuel mixture is controlled so that the air/fuel ratio of the air/fuel mixture becomes leaner than the theoretical air/fuel ratio. On the other hand, in an oxygen absorbed state where the three-way catalyst has absorbed a sufficient amount of oxygen, the air/fuel ratio of the air/fuel mixture is controlled so that the air/fuel ratio of the air/fuel mixture becomes richer than the theoretical air/fuel ratio. That is, in the internal-combustion engine, the air/fuel mixture with the air/fuel ratio leaner than the theoretical air/fuel ratio and the air/fuel mixture with the air/fuel ratio richer than the theoretical air/fuel ratio are alternately formed in the combustion chamber, and the exhaust gas with the air/fuel ratio leaner than the theoretical air/fuel ratio and the exhaust gas with the air/fuel ratio richer than the theoretical air/fuel ratio alternately flow into the three-way catalyst. In the internal-combustion engine, such air/fuel ratio control is executed in order to improve the purification performance of the three-way catalyst by exhibiting the oxygen absorbing and discharging ability of the three-way catalyst.